The gizmo that sits in the distributor where the points used to be in the pertronix setup is called the pickup coil.
As it happens, the rev meter decided to start sticking. The fuel pump is the original type, though new (as of about five years ago). But the way it ‘just stopped’, I’m betting it’s an ignition problem.
That’s what I thought he was talking about, but I wasn’t sure since there is The Coil and I haven’t had a chance to call him or go over to verify.
The engine just stopping g is exactly what happens when an SU pump quits. Very similar to an ignition problem.
Just got the MGB out of the shop on Friday. Saturday I heard a clattering. Tried to drive it to the shop today, and the fan belt broke. 
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Thats a bummer, have you had the shop check that the pulleys are inline? Even typical gas station grade belts should be fine for 6 years. Unless you have something like a fluid leak causing problems I would tend to suspect V-belt drive misalignment.
If someone replaces an alternator with one that is another size, or just puts the washers on the wrong side of the bracket there will be a misalignment and this will produce a lot of heat and dramatically reduce the lifespan.
That is a bit hard to have happen on a B unless one goes to great pains to put on
a different alternator and manufacture brackets and all.
Other alternators dont physically fit, and there are no spacing washers.
Though if you use the wrong profile belt on a B the belt tends to roll over, which does it no favors, if belt vanishes into the pulley, it’s not the right one.
There are a number of ways it can happen, from thicker gaskets on the water pump to a degraded rubber buffer on the harmonic balancer. But the effect would also happen from worn bushing and bearings.
Without images it is hard to tell, but on older car the worn bearings problem tends to be common and people tend to tighten more when they get a bit of squeak from angular misalignment and over tighten the belt which leads to more wear and angular misalignment. That squeak is typically from misalignment and not a loose belt, especially on low power applications like this.
Looking at the vbelt for side wear, and the sheaves for polishing (and using a gauge to make sure they are not worn out) should tell you the cause.
I am not just talking about horizontal alignment, which will sometimes result in belt jumps as you mentioned a broken belt it would tend to point to overheating which also may be cause by just over tightening too.
The one thing that I am 100% sure, based on the length time this is been happening and the number of times it has happened that it is not the belts fault and so must be either to chemical degradation or a problem like misalignment.
I’ve seen this also called the ignition module, like on GM HEI distributors which also had a similar failure mode - when they started to go bad they would fail when they got hot. Let the engine cool and it would start right back up. Changed a number of these on my assorted vehicles in parking lots (usually kept a spare handy).
The pulleys were misaligned back in 2004/5. The mechanic aligned them back when this thread was started.
From what I’ve been reading today, fan belt failure seems to be common on MGBs. It seems people tend to have spares with them at all times.
Oh – I looked at donkeyoatey’s link earlier. I saw ⅝" belts, but the MG needs a ⅜" one.
‘When they start to go bad’? So by replacing the module, it will function normally even when I’m sitting in traffic? How long before it ‘starts to go bad’?
You’re driving a MG and worried about the reliability of a fan belt? 
(said the past owner of an Austin Healey bugeye Sprite, Triumph GT6 and Jensen Interceptor)
Never had a belt fall apart on a B engine.
Later models, have had the air pump seize and lunch that belt, then pump and belt goes bye bye.
Is your B still using the factory ignition module or an after market replacement?
They do go bad, and can work fine until they warm up and then cut out until they cool down, then the car starts again, gets warm and and eventually dies again.
The pickup can go bad but the module be good, or the other way around, which makes testing a pain.
Switching to an after market usually replaces both the module and the pickup
as most of the after markets i have seen use an optical pickup instead of magnetic
though i just prefer to go with points, spare points in the boot tool box are cheap.
Of course, because either the fuel pump, or the lucas electrics will get you long before the belt 
It has a Pertronix electronic ignition. MGBs get hot. It’s just how they are. They must have airflow. If this is going to be a frequent problem (even after changing modules), I may well go back to points.
Just wanted to chime in here with a warning. If you have a car with an “interference” engine, Do not let the serpentine belt break. Be very careful to follow the recommendations in your manual as to mileage before changing.
If you neglect this, and the belt breaks, the valves, way before you can stop the car, will interfere with the pistons and it’s new engine time. All the valves will be either bent or broken, and it will cost in the range of $5,000 - $8,000 to sort out the resulting mess. Not kidding here - our shop gets one of these once in a while.
Right off the top of my head I don’t remember which models of cars is susceptible to this. It will be in your manual.
No need to look it up. I’ve had two belts break on me. 
(FWIW, the B-series engine is an overhead valve design. The camshaft is low, and the valves are operated by pushrods.)
I can tell the heart’s in the right place, but the info is off. Way off.
The accessory drive belt(s), which can be a V-belt like on Johnny’s old MGB or a multi-rib (generally called serpentine) belt like on more recent cars, have no relation to the engine being an interference design or not, and upon breaking will have no effect on the engine’s valves. Obviously it’s a timing belt being thought of here, not a serpentine belt. Yeah, the particular name for the belt is a picky little detail, but it’s a goddamn critical picky little detail, and getting it wrong spreads misinformation.
Now having a timing belt break on an interference design engine will almost always result in one or more bent valves, but on most cars it doesn’t call for an engine replacement, and the fix is likely to be in the 1,000-2,500 range. There may be some expensive-to-fix models that cause pricier damage, but the other 98% won’t cost anywhere near 5,000-8,000 to repair.
I talked to the mechanic this morning, and told him there were spare belts in the boot. They’d been there since I started this thread, and he said ‘Yeah, well they’ve been in there a long time. I’ll get you a new one.’ He said that when a car has been sitting, belts can ‘take a set’ and get weak points.
GARY is right - I should have typed “timing” instead of “serpentine”. My bad.
However, it may be possible to damage only one valve and be able to repair the engine for a thousand or so, but have never seen damage this light. All the ones we get have been basket cases.
The belt will slip. If it has infinite friction where it rides in the pulley grooves, the engine won’t rotate, unless the fan pulley shaft snaps (perhaps possible if things suddenly lock up while the engine is revved, but otherwise the starter won’t be able to crank the engine).
Should not really, not hot enough to damage the ignition.
Its just that heat causes expansion, any heat, and that can show up a broken or failed connection, or in some cases change the resistance in a pickup coil that is already marginal.
But that is nothing MG specific.
I like the points because they could die in the middle of the night in the middle of no where, and i can rectify it with a trip to the glove box
Daylate MGB’s use a timing chain, which will usually slap like hell and make a ton of terrible noise long before it decides to divorce itself from the crank and cam shaft.